Field of the Invention
The invention concerns a method for the actuation of a magnetic resonance imaging scanner for the generation of magnetic resonance image data of an object under examination from whom magnetic resonance raw data are acquired. The invention also relates to an actuation-sequence-determining system. The invention also relates to a magnetic resonance imaging system.
Description of the Prior Art
Modern imaging methods are frequently used to generate two-dimensional or three-dimensional image data, or a time series of image data, which can be used for the visualization of an object under examination. Imaging systems based on magnetic resonance imaging, called magnetic resonance tomography systems, have been successfully established and proven in a multitude of applications. With this type of imaging, generally a static basic magnetic field B0, which is used for initial alignment and homogenization of magnetic dipoles (nuclear spins) to be examined, is superimposed with a rapidly switched magnetic field, called a gradient field, for spatial resolution of the imaging signal. To determine the material properties of an object under examination to be imaged, the dephasing or relaxation time after a deflection of the magnetization out of the initial direction of alignment is determined so that different relaxation mechanisms or relaxation times typical of the material can be identified. The deflection generally takes places by the radiation of a number of RF pulses and, in this case, the spatial resolution is based on a temporally established manipulation of the deflected manipulation of the deflected magnetization with the use of the gradient field in a so measurement sequence or actuation sequence such a sequence defines a precise chronological sequence of RF pulses, the activation of the gradient field (by a switching sequence of gradient pulses) and the acquisition of measurement values. In addition to relaxation, there are other attributes of the nuclear spins that can be detected such as flux measurement and diffusion detection (diffusion imaging).
An association between the measured (detected) magnetization—from which the aforementioned material properties can be derived—and a spatial coordinate of the measured magnetization in the three-dimensional space in which the object under examination is situated, typically takes place with the use of an intermediate step. In this intermediate step, acquired raw magnetic resonance data are entered at readout points in a memory, thereby resulting in a data set known as “k-space”. This entry of data into the k-space memory at respective data entry points is also called “sampling” k-space. The coordinates of the k-space data are encoded as a function of the gradient field. The magnitude of the magnetization (in particular the transverse magnetization determined in a plane transverse to the above-described basic magnetic field) at a specific location of the object under examination can be determined from the k-space data with the use of a Fourier transformation. In other words, the k-space data (magnitude and phase) are required to calculate a signal strength of the signal, and optionally its phase, in three-dimensional space.
Magnetic resonance tomography is a relatively slow type of imaging method because the data are recorded sequentially along trajectories such as, for example, lines or spirals in the Fourier space or k-space. The method for recording images in two-dimensional slices is much less susceptible to errors than recording in three dimensions, because there are fewer encoding steps than with a three-dimensional method. As a result, in many applications, image volumes with stacks of two-dimensional slices are used instead of a single three-dimensional recording. However, due to the long relaxation times of the spins, the imaging times are very long and this, impairs the comfort of a patient to be examined. In addition, during the data recording, patients are not able to leave the magnetic resonance tomography system briefly or even to change their position because a change in position of the patient while data are being acquired would render the imaging process useless, and the entire process would have to be restarted from the beginning. Consequently, an important need is to accelerate the recording of two-dimensional slice stacks.
With another sampling method, selectively excited sub-volumes or part-volumes, so-called “slabs”, are spatially encoded with the use of a three-dimensional sampling method. Once again, there is a need to accelerate the recording speed with this method.
Methods for the acceleration of imaging include, for example parallel imaging techniques. With these known technologies, known by the names “simultaneous multi-slice” (SMS imaging), “slice acceleration” or even “multiband”, numerous slices are excited and read-out simultaneously (see, for example, Breuer et al. MRM 53:684 (2005), Souza et al. JCAT 12:1026 (1988), Larkman et al. JMRI 13:313 (2001), (MRM=Magnetic Resonance in Medicine, JCAT=Journal of Computer Assisted Tomography, JMRI=Journal of Magnetic Resonance Imaging)). For example, with an acceleration factor of 3, in each case, 3 slices are excited and read-out simultaneously. This reduces the required repetition time TR (repetition time TR=time until successive pulse sequences are applied to the same slice) to ⅓ of the required time. Advantageously, these methods reduce the measuring time and/or increase the temporal sampling rate.
There is also a possibility of using “multi-slab” imaging to sample several subvolumes simultaneously in order to accelerate the recording process when sampling selectively excited subvolumes. A procedure of this kind can be considered to be an intermediate stage between 2D multi-slice imaging and complete 3D imaging.
Depending upon the pulse sequence of the sequence to be accelerated, it is not automatically possible to apply all pulses to a number of slices simultaneously without, for example, exceeding SAR thresholds (SAR=specific absorption rate=measure for the absorption of electromagnetic fields in a material) or the available peak power of the RF amplifier. This restriction affects, for example, excitation pulses, i.e. RF pulses, with which spins are manipulated, for example excited or refocused in a specific region of the object under examination
a) with high flip angles, such as, for example, 180° refocusing pulses in the TSE sequence (TSE=turbo spin-echo) or
b) with a high bandwidth, as with spectrally selective excitations or rapid gradient-echo sequences.
In the case of applications in the ultrahigh field range, for example 3 T and higher, in particular 7 T and higher, physiological limits (SAR stress) and/or technical limits (RF peak power) are reached very easily.
If, for example, imaging is initially performed with simultaneous excitation of a number of slices with a first flip angle α1 for all slices, and then further simultaneous excitation is performed with a second flip angle α2 with another value for all slices, this has the drawback that for the MR imaging with the higher flip angle, both the SAR stress and the required RF peak power would be higher by the factor N of the number of simultaneously excited slices. This steep increase means stress and power thresholds are easily exceeded in the case of SMS measurements.
A further problem with SMS imaging relates to the chronological consistency of image data with different contrasts. In the prior art, the spins in the multiple simultaneously excited slices undergo the same contrast evolution. However, if images with different contrast evolutions are required for the diagnosis, these images are conventionally recorded in succession in individual measurements. Although, with simultaneous multi-slice methods, the recording duration of the individual measurement is reduced, there is usually a time interval, which can be as much as several minutes, between the measurements. During this time interval, changes often occur, in particular movements, in the region to be examined. Hence, the assignment of image information between the different contrasts requires complex and error-prone registration methods. A similar problem also occurs if different sequence types are to be applied for different slices. For this application, once again conventionally different sequences are activation in temporal succession.
One possibility for adhering to SAR thresholds is to limit the permitted acceleration factors and hence the number of slices to be read-out simultaneously such that the SAR thresholds or the available RF peak power are not exceeded. However, in many scenarios, it is no longer possible to achieve acceleration factors of 2. Alternatively thereto, there are approaches that, for example, shift the excitation pulses slightly relative to one another in time in order to reduce constructive interference to the excitation pulses. For example, DE 10 2011 082 010 B4 and later Auerbach et al. MRM 69:1261 (2013) suggest a method for a time delay between the application of RF excitations of individual bands. This results in hardly any increase in the required peak power compared to non-accelerated pulses. With this application, the excitation pulses in the individual slices do not differ. Hence, the identical pulse (with the identical flip angle) is slightly time-delayed at another position. However, the latter instance represents a restriction of the method since both slices initially undergo slightly different dephasing, which has to be taken into account by suitable compensation measures (for example considered during the application of a refocusing pulse).
Norris et al. MRM 66:1234 (2011) suggests an approach known as PINS (“power independent of number of slices”) in order to limit the SAR exposure. This approach is based on convolution of the excitation pulse with a Dirac comb function. This achieves a periodic repetition of the slice profile, wherein, however, the SAR exposure is independent of the number of excited slices and greatly reduced compared to the standard simultaneous multi-slice approach. One drawback of this technique is that the recurring excitation pattern continues ad infinitum and hence, depending upon the positioning of the image volume, (unwanted) more remote anatomical structures are also excited. In addition, a prolongation of the RF pulse duration should be expected with the practical application of this method.